This invention relates to force responsive transducers and particularly to capacitive transducers utilizing one or more capacitive elements to measure inputs with high precision, but also provides low cost manufacturing methods.
Numerous capacitive transducers are well known in the art. There are two classes of ceramic capacitive transducers. The first consists of devices that have two-dimensional structures located in a spaced apart manner, with a peripheral seal joining them together to form a capacitive element. This group of transducers is represented by the patents to Polye, U.S. Pat. Nos. 3,634,727 and 3,858,097, the patents to Bell et al, U.S. Pat. Nos. 4,151,578, 4,177,496 and 4,207,604, and the patent to Dias et al, U.S. Pat. No. 4,064,550. These transducers work best at low to middle range pressures (1 psi and 200 psi). At higher pressures, their peripheral seals are subjected to high tensile stresses which can cause catastrophic failure. Even before this point is reached, these two-dimensional designs provide poor results because of the inherently high stress levels. Most of these transducers also suffer poor nonlinearity performance since their two-dimensional nature leads to a fourth order deflection curve. In particular, the Bell et al U.S. Pat. No. 4,177,496 provides terminal based nonlinearities of greater than 2%. It also is limited to higher ranges, above 2 psi, because of the great difficulty in handling parts of 10 mils or less thickness in a manufacturing environment. In order to utilize this design below 2 psi, a significantly greater diameter, smaller diaphragm thickness, and/or higher electronic gain is required. All such solutions increase cost and reduce commercial viability.
The second class of transducers consists of transducers with movable spaced apart reference plates such as described in the patent to Lee et al U.S. Pat. No. 3,859,575 and the patent to Bell U.S. Pat. No. 4,295,376. In Lee's construction the deflectable diaphragm comprises the outer circumferential portion of the end cap of a hollow metal cylinder which can be threaded with a base structure which provides support for a spaced apart reference plate. The Bell transducer uses a novel, movable reference plate to provide a differential capacitive output, e.g., when the diaphragm deflects one capacitive increases in value and the other decreases in value. This design also incorporates torsional suspension elements to reduce the bending stresses in the joint that attaches and spaces apart the reference plate from the diaphragm. Unfortunately, these spacer elements are still highly stressed and may fail under high pressure and/or vibration.
Hence there is a need for new devices that offer better linearity, lower stress levels and resistance to vibration and shock, as well as being capable of measuring pressures up to 5000 psi. Such new device should offer stability over a wide temperature range, be amenable to low cost manufacturing techniques, such as thick film processing, and provide reliable performance over a long period of time.
One particular application is the vehicular field, where pressures from 200 psi to several thousand psi are to be monitored by the on-board computers now responsible for management of the engine and other vehicular systems such as transmission and hydraulic braking systems. The problem is to provide precise, linear inputs to these on-board computers in a cost effective way.
Another important application is in the field of process control wherein many measurements are made of fluid pressures, flows and levels in order to optimize throughput and increase the quality of the final product. Most of these "static" pressures are in the 100 psi to 3000 psi range.